This invention relates to an improved finishing process for nonwoven sheets of polyethylene film-fibrils. In particular, this invention relates to a process for preparing a nonwoven, polyethylene, film-fibril sheet which has high delamination strength and uniformity of appearance.
Nonwoven sheets of polyolefin film-fibrils, particularly polyethylene, are utilized for book binding as a replacement for cloth which is more expensive. For this application, the nonwoven sheet should be printable, opaque, uniform in appearance, and durable, i.e., have high delamination strength and high abrasion resistance. Nonwoven, polyolefin, film-fibril sheets formed by deposition of continuous fibrillated strands onto a moving belt can be compressed and/or fused, e.g. by calendering or by hot air treatments, to develop further certain desirable qualities. However, a sheet treated in such manner tends to have non-uniform translucent areas. Efforts to achieve uniformity of appearance simultaneous with the other qualities desired in a non-woven sheet to be used as a book cover have led to considerable research and have been the subject of several patents.
U.S. Pat. No. 3,442,740, issued to David on May 6, 1969, discloses a finishing process for making a nonwoven sheet which has uniform high opacity, a high degree of flatness, high surface stability, and high delamination resistance. The disclosed process comprises thermally bonding a film-fibril sheet by subjecting it in a heating zone to light compression between two surfaces thereby preventing shrinkage, the first surface being a hard heat-conducting material and being maintained at a temperature substantially equal to or greater than the upper limit of the melting range of the film-fibril elements throughout the treatment, the second surface being a flexible, poor heat conductor, the film-fibril sheet being exposed long enough to allow the face of the sheet exposed to the hard heat conducting material to reach a temperature within 7.degree. C of the upper limit of the melting range of the film-fibril elements, but not substantially above said upper limit, and to allow the second face of the sheet to reach a temperature 0.8.degree. to 10.degree. C lower than the first face of the sheet, finally directly passing the sheet while under restraint through a cooling zone wherein the temperature of the film-fibril sheet throughout its thickness is reduced to a temperature less than that at which the sheet will distort or shrink when unrestrained.
It was later found that while the process of the David patent provides sheets with good uniform opacity, this quality seemed to be somewhat dependent upon the history of the sheet prior to bonding. U.S. Pat. No. 3,536,552, issued to Lee on Oct. 27, 1970, is directed to this problem and discloses a process which comprises providing a lightly consolidated film-fibril sheet having a density of 0.11 to 0.26 g/cm.sup.3, a basis weight of 10 to 200 g/m.sup.2 and a coefficient of variation in basis weight of less than 0.15 (15%); passing this sheet through a constant clearance nip between a pair of hard calender rolls at substantially room temperature, said clearance being adjusted to provide a new sheet thickness between 0.5 and 0.85 of the thickness of the lightly consolidated film-fibril sheet; passing the sheet through a heating zone, while under restraint to prevent shrinkage, and raising the temperature of either face of the sheet to within 7.degree. C of the upper limit of the melting range of the film-fibrils but not substantially above said upper limit; then cooling the sheet while under restraint to a temperature below that at which the sheet distorts or shrinks.
The process disclosed by the Lee patent requires rather stringent control of the basis weight uniformity, i.e., coefficient of variation of less than 0.07 (7%), during production of the lightly consolidated sheet in order to obtain a starting sheet capable of being thereby finished into a sheet having the high delamination strength, as defined later herein, while still having uniformity of appearance. Moreover, it has been found that often as productivity is increased, the basis weight uniformity diminishes. A process which could utilize nonwoven sheets displaying a somewhat lesser uniformity of basis weight would greatly alleviate the conflicting needs for greater uniformity and greater productivity.